The invention relates to a method for reducing the effects of noise.
Radio receivers employ various equalizers for reducing inter-symbol interference. Inter-symbol interference is due to linear or non-linear distortions caused to a signal on the radio channel. For example, inter-symbol interference, or ISI, appears when a signal spreads outside the desired time slot. In addition, equalizers are used to reduce noise and interference in the signal.
TDMA (Time Division Multiple Access) systems employ MMSE-DFE equalizers (Minimum Mean-Square Error Decision Feedback Equalizer) to reduce interference. However, the MMSE-DFEs in use do not always produce an optimal result. In addition, the equalizers are most complex to implement and to use in practical applications. In connection with use, complexity means that large matrices have to be processed.
Another problem is encountered when MLSE (Maximum Likelihood Sequence Estimation) or MAP (Maximum a posteriori probability) equalizers are used in connection with multi-level modulations and great channel impulse response delay spreads. Such cases require computing, which increases the number of states needed and also the amount of computation.
An increasing amount of computation is required for determining coefficients for equalizers, but current calculation methods are not very well suited for use when intensive computation is needed. At least in some cases the structures required by the calculation complicate the receiver structure too much. For example, complex computation is needed for computing the tap coefficients of filters used in a receiver. The computation requires large-dimension matrices to be used which slow down the calculation and make it more complicated.
It is therefore an object of the invention to provide a method and receiver that allow the above-mentioned problems to be solved. This is achieved with a method disclosed in the preamble, characterized by taking samples from a received signal, the noise in the samples being converted to white noise; filtering each sample signal comprising white noise in a filter; and computing, prior to the filtering of a sample signal, filter tap coefficients from the sample signal for the filter filtering the sample signal to be supplied to the filter; summing the filtered sample signals in groups to provide first summed signals such that each first summed signal comprises a sample signal filtered in at least one filter; summing the first summed signals to provide the actual summed signal, which is detected.
This is also achieved with a method disclosed in the preamble, characterized by forming sample signals by taking samples from a received signal, the noise in the samples being converted to white noise; dividing signals comprising white noise among a plural number of signal groups; filtering sample signals of each signal group in a signal-group-specific filter; and detecting a signal formed of at least one signal of each signal group comprising filtered signals.
The invention further relates to a receiver for reducing the effects of noise.
The receiver of the invention is characterized in that the receiver comprises at least one sampling means for producing sample signals from a received signal; a whitening means for converting the noise in the sample signals to white noise; digital filters for filtering sample signals comprising white noise; a computation means for computing filter tap coefficients for the filter from the sample signal; summers, each one of which is arranged to receive a filtered signal from at least one filter, the summers being arranged to sum the signals they receive from the filters to produce first summed signals; and a detector for summing the first summed signals to produce the actual summed signal and to detect the actual summed signal.
A receiver of the invention is also characterized in that the receiver comprises at least one sampling means for producing sample signals from a received signal; a whitening means for converting the noise in the sample signals to produce white noise and for dividing signals comprising white noise among a plural number of signal groups; a signal-group-specific digital filter for filtering the sample signals in the signal group; and a detector for detecting a signal formed of at least one signal of each signal group comprising filtered signals.
The preferred embodiments of the invention are disclosed in the dependent claims.
An underlying idea of the invention is that the noise in the sample signals is converted to white noise and that, before the sample signal is filtered, filter tap coefficients are calculated from the sample signal comprising white noise for the filter filtering the sample signal.
The method and receiver of the invention provide various advantages. The method allows for a more simplified equalizer implementation. In addition, the computation of filter tap coefficients is easier and speedier than with previous methods since the noise is whitened before the signal is supplied to the filter and before the tap coefficients are computed. The method thus allows matrices of relatively small dimensions to be used for the computation of tap coefficients. Moreover, the method allows to reduce the number of taps in the filter, which further facilitates the computation.
Yet another advantage of the invention is that receiver structure is simplified, which means that a receiver may be implemented using less equalizers than in prior art receivers. In addition, the method of the invention allows the interference tolerance of the receiver to be improved. With the solution of the invention, the performance of the equalizer can be optimized. This means that although the solution of the invention provides low equalizer complexity, nevertheless, the performance of the equalizer can be kept at a good level.